From: Jann Horn jannh@google.com
commit 6e7e63cbb023976d828cdb22422606bf77baa8a9 upstream.
When check_xadd() verifies an XADD operation on a pointer to a stack slot containing a spilled pointer, check_stack_read() verifies that the read, which is part of XADD, is valid. However, since the placeholder value -1 is passed as `value_regno`, check_stack_read() can only return a binary decision and can't return the type of the value that was read. The intent here is to verify whether the value read from the stack slot may be used as a SCALAR_VALUE; but since check_stack_read() doesn't check the type, and the type information is lost when check_stack_read() returns, this is not enforced, and a malicious user can abuse XADD to leak spilled kernel pointers.
Fix it by letting check_stack_read() verify that the value is usable as a SCALAR_VALUE if no type information is passed to the caller.
To be able to use __is_pointer_value() in check_stack_read(), move it up.
Fix up the expected unprivileged error message for a BPF selftest that, until now, assumed that unprivileged users can use XADD on stack-spilled pointers. This also gives us a test for the behavior introduced in this patch for free.
In theory, this could also be fixed by forbidding XADD on stack spills entirely, since XADD is a locked operation (for operations on memory with concurrency) and there can't be any concurrency on the BPF stack; but Alexei has said that he wants to keep XADD on stack slots working to avoid changes to the test suite [1].
The following BPF program demonstrates how to leak a BPF map pointer as an unprivileged user using this bug:
// r7 = map_pointer BPF_LD_MAP_FD(BPF_REG_7, small_map), // r8 = launder(map_pointer) BPF_STX_MEM(BPF_DW, BPF_REG_FP, BPF_REG_7, -8), BPF_MOV64_IMM(BPF_REG_1, 0), ((struct bpf_insn) { .code = BPF_STX | BPF_DW | BPF_XADD, .dst_reg = BPF_REG_FP, .src_reg = BPF_REG_1, .off = -8 }), BPF_LDX_MEM(BPF_DW, BPF_REG_8, BPF_REG_FP, -8),
// store r8 into map BPF_MOV64_REG(BPF_REG_ARG1, BPF_REG_7), BPF_MOV64_REG(BPF_REG_ARG2, BPF_REG_FP), BPF_ALU64_IMM(BPF_ADD, BPF_REG_ARG2, -4), BPF_ST_MEM(BPF_W, BPF_REG_ARG2, 0, 0), BPF_EMIT_CALL(BPF_FUNC_map_lookup_elem), BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, 1), BPF_EXIT_INSN(), BPF_STX_MEM(BPF_DW, BPF_REG_0, BPF_REG_8, 0),
BPF_MOV64_IMM(BPF_REG_0, 0), BPF_EXIT_INSN()
[1] https://lore.kernel.org/bpf/20200416211116.qxqcza5vo2ddnkdq@ast-mbp.dhcp.the...
Fixes: 17a5267067f3 ("bpf: verifier (add verifier core)") Signed-off-by: Jann Horn jannh@google.com Signed-off-by: Alexei Starovoitov ast@kernel.org Link: https://lore.kernel.org/bpf/20200417000007.10734-1-jannh@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- kernel/bpf/verifier.c | 28 ++++++++++----- tools/testing/selftests/bpf/verifier/value_illegal_alu.c | 1 2 files changed, 20 insertions(+), 9 deletions(-)
--- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -1866,6 +1866,15 @@ static bool register_is_const(struct bpf return reg->type == SCALAR_VALUE && tnum_is_const(reg->var_off); }
+static bool __is_pointer_value(bool allow_ptr_leaks, + const struct bpf_reg_state *reg) +{ + if (allow_ptr_leaks) + return false; + + return reg->type != SCALAR_VALUE; +} + static void save_register_state(struct bpf_func_state *state, int spi, struct bpf_reg_state *reg) { @@ -2056,6 +2065,16 @@ static int check_stack_read(struct bpf_v * which resets stack/reg liveness for state transitions */ state->regs[value_regno].live |= REG_LIVE_WRITTEN; + } else if (__is_pointer_value(env->allow_ptr_leaks, reg)) { + /* If value_regno==-1, the caller is asking us whether + * it is acceptable to use this value as a SCALAR_VALUE + * (e.g. for XADD). + * We must not allow unprivileged callers to do that + * with spilled pointers. + */ + verbose(env, "leaking pointer from stack off %d\n", + off); + return -EACCES; } mark_reg_read(env, reg, reg->parent, REG_LIVE_READ64); } else { @@ -2416,15 +2435,6 @@ static int check_sock_access(struct bpf_ return -EACCES; }
-static bool __is_pointer_value(bool allow_ptr_leaks, - const struct bpf_reg_state *reg) -{ - if (allow_ptr_leaks) - return false; - - return reg->type != SCALAR_VALUE; -} - static struct bpf_reg_state *reg_state(struct bpf_verifier_env *env, int regno) { return cur_regs(env) + regno; --- a/tools/testing/selftests/bpf/verifier/value_illegal_alu.c +++ b/tools/testing/selftests/bpf/verifier/value_illegal_alu.c @@ -88,6 +88,7 @@ BPF_EXIT_INSN(), }, .fixup_map_hash_48b = { 3 }, + .errstr_unpriv = "leaking pointer from stack off -8", .errstr = "R0 invalid mem access 'inv'", .result = REJECT, .flags = F_NEEDS_EFFICIENT_UNALIGNED_ACCESS,